Many industries use thin walled structures joined by fasteners. For example, thin-walled metal panels joined by rivets form the skin of an aircraft and other vehicles. Routine operation of the aircraft subjects the skin to harsh environments that includes large temperature changes and high stress. As a result, cracks in the skin can form. Cracks at or near the rivets can also form during manufacturing of the aircraft. Nondestructive methods for detecting cracks in these structures include radiography, eddy current, and ultrasonic methods. These methods, however, suffer from several problems.
Cracks in a countersink of a rivet, for example, cause particular problems in the aircraft industry. Radiography can be used to detect these cracks, but the technique is limited to detection of large cracks. Eddy currents can detect cracks that extend at least 100 mil past the rivet head, however, for detection of smaller cracks, particularly in the countersink, the fasteners need to be removed first. Eddy current methods are also limited to conductive materials. Ultrasonic methods, using a phased array, can detect smaller sized cracks in the countersink without first removing the fastener. However, this technique requires surface preparation and a contact media. Ultrasonic methods are also time consuming and expensive. Laser generated interferometry techniques also exist, but the low intensity waves that are generated are difficult to measure in an industrial environment. Another drawback is that each of the above methods requires a highly trained operator to conduct the testing and analyze the results.
An improved method and system for detecting cracks at and near a fastener is desired.